Ethylene-vinyl alcohol copolymers (hereinafter, may be abbreviated as EVOH(s)) are polymeric materials that are superior in barrier properties against gas such as oxygen, oil resistance, antistatic properties, and the like. Therefore, EVOH-containing resin compositions have been widely used as various types of packaging materials, etc., through molding into containers, films, sheets and the like. Particularly, in the field of blow molded containers and the like, multilayer structures have been widely employed which are formed from a layer constituted with the EVOH-containing resin composition, a layer constituted with other thermoplastic resin that is superior in moisture resistance, impact resistance, etc., and the like.
For the blow molded containers, melt molding is generally conducted; however, the melt molding may result in impaired appearance characteristics of molded articles through generation of defects such as gels, seeds (dirt under paint), streaks and the like. In addition, the gels, seeds and the like not only lead to impairment of the appearance of the molded article but also cause deterioration of performance; therefore, suppression of these events is needed.
Furthermore, in the step of manufacturing blow molded containers, an operation of the melt molding apparatus must be stopped once and restarted after a certain time period for a change of the resin, a halt in manufacture on weekends, etc., as well as a replacement operation of the molding die, and the like. In these instances, a small amount of the resin composition remaining in the apparatus may be deteriorated during the steps of elevation and lowering of the temperature of the apparatus, and gelled upon restarting, leading to the occurrence of a disadvantage of causing deterioration of appearance characteristics and qualities such as strength of the molded articles. Therefore, after the restarting of the apparatus, it is necessary to conduct idling for a certain time period to await until such gels and the like disappear and the appearance of the molded articles is reverted to the normal state. In light of the production cost, the time period required for the reverting is preferably short, in other words, a self-purge feature is preferably favorable; however it is impossible to conclude that the self-purge feature of conventional EVOH-containing resin compositions is favorable.
Therefore, as EVOH-containing resin compositions having an improved self-purge feature, (1) a resin composition prepared by adding a polyolefin resin, a carboxylic acid-modified polyolefin resin and an alkaline earth metal salt of a lower fatty acid having 9 or less carbon atoms to an EVOH resin at a specific ratio (Japanese Unexamined Patent Application, Publication No. H5-255554), and (2) an EVOH-containing resin composition containing a carboxylic acid having a specific viscosity and a molecular weight of less than 75, and an alkaline earth metal salt at a specific ratio (Japanese Unexamined Patent Application, Publication No. 2001-234008) have been known. However, due to containing the alkaline earth metal, these EVOH-containing resin compositions may cause coloring of molded articles, and also the self-purge feature is not satisfactory. Particularly, the aforementioned EVOH-containing resin composition (1) is disadvantageous in leading to deterioration of the hue and impairment of the long-run workability, owing to the addition of the polyolefin resin.
In addition, a production method of EVOH has been known in which crotonaldehyde is coexisted in a polymerization step of ethylene and vinyl acetate (Japanese Unexamined Patent Application, Publication No. 2007-31725). According to this production method, an adhesion of scales in the interior of a polymerization tank can be suppressed by the coexistence of crotonaldehyde during polymerization. As a result, in regard to films of EVOH produced by this production method, generation of fish eyes resulting from the scales detached and contaminated in the polymer can be reportedly decreased.
However, in the production method, crotonaldehyde added in the polymerization is consumed in the polymerization step and a saponification step, and further washed away with water in a washing step and the like; therefore, crotonaldehyde hardly remains in a finally obtained resin composition containing EVOH since the solubility of crotonaldehyde in water is 18.1 g/100 g at 20° C. (see THE MERCK INDEX 14th 2006). Therefore, effects of the unsaturated aldehyde such as crotonaldehyde on an improvement of thermal stability and/or long-run workability in a secondary processing (melt molding, etc.) of the resultant EVOH has been unknown.